1. Field
The present invention generally relates to AC to DC front end power supplies with high power factor (PF) and low total harmonic distortion (THD) requirements and, more particularly, to electronic ballasts for powering gas discharge lamps, and especially for electronic ballasts dimmed by TRIAC based wall dimmers.
2. Description of Related Art
Discharge lamps create light by exciting an electrical discharge in a gas that can create visible light. In the case of fluorescent lamps the gas is a mixture of argon, krypton and/or neon, plus a small amount of mercury. Other types of discharge lamps may use other gasses. The gas is contained in a partially evacuated transparent virtuous envelope called a bulb or arc tube depending upon the type of lamp.
In conventional lamps conductive electrodes mounted inside the bulb or arc tube along with the gas provide the electric field used to drive the discharge.
Use of electrodes creates certain problems. First, the discharge has to be designed to have a relatively high voltage in order to minimize loses at the electrodes. In the case of fluorescent lamps, this leads to long, thin lamps, which are ideal for lighting office ceilings, but are not a good fit for lamps designed to replace conventional incandescent lamps. Fluorescent lamps designed to replace incandescent lamps are constructed by bending the long, thin tube into multiple parallel tubes or into a spiral, which is now the most common form of CFLs. A plastic cover shaped like a conventional incandescent lamp is sometimes placed over the bent tubes to provide a more attractive shape, but these covers absorb light, making the lamp less efficient. Bent and spiral tube lamps also have wasted space between the tubes, making them larger than necessary. The use of a cover increases the size further.
The use of electrodes creates problems other than shape and size. Electrodes will wear out quickly if the lamp is turned on and off many times, as it would be in a residential bathroom. The life of the electrodes can also be reduced if the lamp is dimmed, because the electrodes must be maintained above a specific temperature for proper operation and operation at lower power allows the electrodes to cool.
In addition, the long thin shape required to use electrodes increases the time required for mercury vapor to diffuse from one part of the tube to another, leading to the long warm-up times we associate with many compact fluorescent lamps.
Finally, the electrodes must be chemically compatible with the gas used in the lamp. While this is not a concern with typical fluorescent lamps, it can be a problem with other types of discharge lamps.
The best way to avoid the problems caused by electrodes is to make a lamp that does not use electrodes, a so-called electrodeless lamp. In an electrodeless lamp, the discharge is driven by one of 1) an electric filed created by electrodes mounted outside the bulb or arc tube; or 2) an electric filed created by a very high frequency electromagnetic field, usually in combination with a resonant cavity, and 3) an electric field created by a high frequency magnetic field without the use of a resonant cavity. This latter lamp is called an induction-coupled electrodeless lamp, or just “induction lamp.”
In an induction lamp, a high frequency magnetic field is used to create the electric field in the lamp, eliminating the need for electrodes. This electric field then powers the discharge.
Since induction lamps do not use electrodes, they do not have to be built into long thin tubes. In fact, a ball-shaped bulb, such as the bulb used for conventional incandescent lamps, is an ideal shape for an induction lamp. In addition, since induction lamps do not use electrodes, they can be turned on and off frequently with no loss of life. The absence of electrodes also means that induction lamps can be dimmed with no reduction in lamp life. Finally, the ball-shaped lamp envelope allows rapid diffusion of mercury vapor from one part of the lamp to another. This means that the warm-up time of induction lamps is faster than the warm-up time of most conventional compact fluorescent lamps.
Induction lamps fall into two general categories, those that use a “closed” magnetic core, usually in the shape of a torus, and those that use an “open” magnetic core, usually in the shape of a rod. Air core induction lamps fall into this latter category. Closed core lamps can be operated at frequencies generally above 50 kHz, while open core lamps require operating frequencies of 1 MHz and above for efficient operation. The lower operating frequency of closed core induction lamps makes them attractive; however, the bulb design required to accommodate the closed core makes them generally unsuitable for replacing standard in incandescent lamps. Open core induction lamps, while requiring higher operating frequencies, allow the design of lamps that have the same shape and size as common household incandescent lamps. This application is addressed to open core induction lamps.
In spite of their obvious advantages, there are very few open core induction lamps on the market today. One reason for the lack of commercially successful products is the cost of the high frequency ballast. Conventional fluorescent lamps, including CFLs, can be operated at frequencies from 25 kHz to 100 kHz, a frequency range where low cost ballast technology was developed in the 1990s'; and closed core induction lamps can be operated at frequencies from 50 kHz to 250 kHz, which is only slightly more expensive. However, open core induction lamps require operating frequencies of 1 MHz or higher. The United States Federal Communications Commission (FCC) has established a “lamp band” between 2.51 MHz and 3.0 MHz that has relaxed limits on the emission of radio frequency energy that may interfere with radio communication services. Cost effective open core induction lamps should therefore have an operating frequency of at least 2.51 MHz.
The lack of commercially successful open core induction lamps can be traced to the failure to develop a low cost ballast that can operate in the 2.51 MHz to 3.0 MHz band while meeting all the requirements of the FCC, is small enough to fit into a ballast housing that has the same size and shape as a conventional incandescent lamp, and can be dimmed on conventional TRIAC dimmers found in homes in the U.S. The present disclosure addresses one or more of these issues.
Passive Valley Fill Circuits are widely used in front end power supplies of electronic ballasts for power factor (PF) correction and reduction of total harmonic distortion (THD). The advantages of these front end power supplies include their low cost, high efficiency and simplicity, features that are desirable for low power compact fluorescent lamps (CFLs). However, the PF of electronic ballasts with Passive Valley Fill Circuits is still lower and THD is higher than electronic ballasts that employ active power factor correctors utilizing boost converters. Electronic ballasts with Passive Valley Fill power factor circuits are used in high efficiency lamps intended to replace incandescent lamps in applications where those lamps are powered and dimmed by TRIAC-based wall dimmers. For operation with TRIAC-based wall dimmers it is desirable to have sufficient discharge capacitance in the Passive Valley Fill circuit in order to maintain the gas discharge in the lamp during line current interruptions. It is also understood that the power factor is improved and the dimming range may be extended as the minimum voltage of the Passive Valley Fill circuit is reduced. Therefore, room still exists for improvements in electronic ballasts with Passive Valley Fill circuits.